The present invention relates to analog and digital circuits. In particular, the present invention relates to a method and apparatus for combining trim and temperature trip point for a circuit.
Accurate and stable components are needed for a wide variety of analog and digital applications. Some of these components include bandgap voltage references, temperature sensors and digital to analog converters (DACs). While each of these components is designed to be accurate each may vary slightly from the specifications. Trimming techniques are used to try to alleviate some of these discrepancies. Trimming many components, however, increases the complexity of the circuit.
Bandgap voltage references ideally provide a voltage having a zero temperature coefficient. The actual temperature characteristic, however, is a parabolic temperature curve due to nonlinearities of the temperature behavior of the transistors forming the bandgap, as well as to nonlinearity of the circuit resistance temperature coefficient. As a result, a bandgap voltage reference may not output the exact reference voltage as specified.
Similarly, the requirements and stability with respect to temperature sensors very from one sensor to the next. Variations in the transistor base-to-emitter voltages from lot to lot cause each sensor to measure the same temperature slightly differently.
Additionally, the temperature set point may vary due to manufacturing variables such as sheet resistance variations and threshold voltage matching errors and resistor matching errors. Further, some devices require many components for set point programming.
The present invention is directed at addressing the above-mentioned shortcomings, disadvantages and problems, and will be understood by reading and studying the following specifications.
In accordance with aspects of the present invention, an apparatus and method are provided combining bandgap trim, temperature sensor trim, comparator offset trim, and the setting of the temperature trip point.
In one aspect of the invention, a comparator compares a programmable reference signal and temperature signal and trips the circuit based on the comparison. In accordance with this aspect, the temperature signal is combined with a programmable reference signal to trim the circuit and set trip point. The programmable reference signal may be adjusted until the circuit trips at the desired temperature.
Another aspect of the invention includes a method for setting a temperature trip point for a circuit. A programmable offset signal generator is programmed to generate a program signal to trip the circuit at the desired trip temperature. The program signal trims a temperature sensor and a signal reference and a comparator offset.
Yet another aspect of the invention includes calculating slope from a single point measurement. In accordance with this aspect, a universal point is determined. The universal point is a constant value for all devices at absolute zero. The single point measurement is taken at a predetermined temperature. The slope is then calculated from the universal point and a single measurement.